Gears are widely used in a mechanical transmission device (such as a watch, a mechanical equipment, etc.) to transmit torques, to change rotational directions, or to change rotational speeds, etc.
Referring to PIG. 1, a conventional gear transmission device 10 includes a driving gear 11 and a plurality of driven gears 12 meshing with the driving gear 11. Each of the driving and driven gears 11, 12 has a main body 111, 121 and a plurality of teeth 112, 122 formed on and extending radially and outwardly from the main body 111, 121. Rotation of the driving gear 11 results in rotation of each driven gear 12 in an opposite rotational direction. Hence, the driven gears 12 cannot be disposed to mesh with each other, which impose a limit on the transmission of the device 10 and the device 10 may not be able to meet requirements for different applications. For example, to provide the driven gears 12 with rotation in different rotational directions, an extra gear (not shown) is needed to interconnect two of the driven gears 12, which increases the manufacturing cost of the gear transmission device 10 and renders the gear transmission device 10 bulky.